1. FIELD OF THE INVENTION:
This invention relates to signal frequency measurement devices and more particularly to Instantaneous Frequency Measurement (IFM) receivers intended for measuring and reporting frequencies of radar and other signals.
2. DESCRIPTION OF THE PRIOR ART:
The instantaneous frequency measurement (IFM) receiver using delay line phase discriminators was developed in the '50s and '60s as a wide bandwidth and high frequency resolution analyzer of pulsed radar signals. Accurate real-time indication of the frequency of pulsed or continuous wave (CW) radar signals is an essential link in electronic warfare (EW) systems. Equipment using the well-known IFM technique can provide frequency information in such a short time that highly effective jamming signals may be produced and transmitted in response to individual pulses received. The IFM receiver, because of its desirable characteristics, has become a major EW building block over the past 30 years. In the last 20 years, novel means of digitizing the angle output of the phase discriminators have given us the digital IFM (DIFM) which is now dominant commercially.
In a typical system, the received radar signal is sent through a delay line of delay r. This causes the signal to undergo a phase shift .phi. which is directly related to the frequency f of the signal according to .phi.=2.pi.f.tau.. If the phase shift is measured, the frequency of the signal can be measured and reported. The IFM receiver is able to measure the frequency of a single pulse of signal as short as 100 nanoseconds or less. Thus, the frequency measurement is referred to as "instantaneous."
However, in all such IFMs and DIFMs, multiple signals received simultaneously present serious problems. Simultaneous signals are received due to random pulse coincidence, simultaneous multi-frequency pulsed radars, CW or high duty-cycle radars, multipath, or spurious receiver-generated components. Although several patented solutions to special cases of this problem have appeared throughout the '80s, the general problem is widely considered unsolved.
The earliest pertinent prior art is described in U.S. Pat. No. 2,434,914, granted to C. W. Earp in 1948 for Frequency Indicating Cathode-Ray Oscilloscope. This patent established the fundamental idea behind the Instantaneous Frequency Measurement or IFM receiver. The description is of an apparatus which subjects a sinusoidal signal of unknown frequency to a known delay so that the signal and its delayed version may be applied to the x and y axes, respectively, to deflect the trace of a cathode ray tube (CRT) oscilloscope. It is shown there that, while the signal is `ON,` the oscilloscope trace is deflected from the center of the oscilloscope face to a position with radius proportional to signal strength and angle proportional to frequency. That is, it measures virtually instantaneously the frequency of the incoming signal; hence its description as an IFM receiver. Frequency is measured by visually interpreting the angle of the deflected oscilloscope trace.
If two or more signals appear simultaneously on the input, the trace cycles between the `two correct positions` at the difference frequency, tracing out a complicated path which depends on the signal frequencies and amplitudes. The occurrence of two or more signals simultaneously causes a sporadic dance of the signal, displayed as a spot, all around the screen, and an inability of the computer to make any consistent interpretation of the data. A means for extracting the two frequencies from such observations has not been available.
The first example of a multichannel, digital output IFM appeared in U.S. Pat. No. 3,189,820, granted to R. V. Lowman in 1965 for Plural Channel Signal Receiver including Signal Delay Means. It was thus possible to digitize the angle of the position of the spot on the scope face to provide the correct half-bandwidth of the signal frequency; 1 bit. Several such channels with different delays could be used to provide additional bits of the signal frequency.
Another example of such a multichannel digital IFM appeared in the U.S. Pat. No. 3,617,900 granted to C. Fink, F. E. Burnham and M. I. Marks in 1971 for a Digital Frequency Detecting System. Thus, it became possible to measure the frequency digitally to the correct 90.degree. quadrant (2 bits); even correct sectors of 5.625.degree. (6 bits) had become practical. Several such channels utilizing different delays can then be incorporated with angle detection in each channel for high resolution frequency of a single signal. However, the method is intended to measure only a single frequency.
Still another example of a multichannel digital IFM or digital frequency discriminator (DFD) appeared in the U.S. Pat. No. 4,188,628 for Frequency Measuring Circuit in a Passive Radar Receiver granted to H. B. Langeraar and G. V. Van Rooijen in 1980. Multiple delay line discriminators, which are tuned to different frequency bands, perhaps overlapping, are used with digital detection and recombination filters to select `preferred` discriminator outputs. However, the method is intended to measure only a single frequency.
It is also possible to utilize a chain of frequency dividers and a delay line frequency discriminator as described in U.S. Pat. No. 4,859,934 granted to P. M. Gale, M. McMillan and
A. Gagnon in 1989 for Apparatus for Measuring the Frequency of Microwave Signals. The method is intended to measure only a single frequency.
A particular simultaneous signals problem arises in the case of a radar which transmits two frequencies simultaneously. An attempt to address this problem appeared in U.S. Pat. No. 3,939,411 granted to W. G. James in 1976 for Instantaneous Frequency Measurement System. The idea is that a dispersive delay line will transform simultaneous pulses into time sequential pulses for input to an IFM receiver. This method fails in the case of two pulses which happen to transform into time coincident pulses by the dispersive delay line. Also, it is not intended for the pulse-on-CW case.
A means to measure frequencies of continuous-wave (CW) signals and pulses appeared in U.S. Pat. No. 4,194,206 granted to J. B-Y. Tsui and G. H. Schrick in 1980 for Instantaneous Frequency Measurement (IFM) Receiver with Capability to Separate CW and Pulsed Signals. This works for pulse-on-CW but is not intended for pulse-on-pulse.
An apparatus to detect the presence of two or more signals during the critical frequency encode time in the IFM receiver as a DATA-NOT-VALID flag appeared in U.S. Pat. No. 4,336,541 granted to J. B-Y. Tsui, R. L. Shaw and J. Caschera in 1982 for Simultaneous Signal Detector for an Instantaneous Frequency Measurement Receiver. The circuit described therein will detect and indicate simultaneous signals when the leading edges of the two pulses are separated by more than 20 nanoseconds. But when the two leading edges are closer, the detection circuit may fail to report simultaneous signals. The circuit treats overlapping but not simultaneous pulses.
An apparatus to detect the presence of two or more simultaneous pulses appeared in U.S. Pat. No.4,426,648 granted to J. B-Y. Tsui and R. L. Shaw in 1984 for Simultaneous Signal Detection for IFM Receivers by Detecting Intermodulation Products. An apparatus to detect the presence of two or more simultaneous signal pulses appeared in U.S. Pat. No. 4,547,727 granted to J. B-Y. Tsui and R. L. Shaw in 1985 for Simultaneous Signal Detection for IFM Receivers by Transient Detection. These provide a simultaneous signal warning flag-they do not measure frequency.
Another simple IFM implementation utilizing only two delay line discriminators and a method for calculating frequency based on the "Chinese Remainder Theorem" appeared in U.S. Pat. No. 4,963,816 granted to J. B-Y. Tsui and W. S. McCormick in 1990 for Instantaneous Frequency Measurement (IFM) Receiver with Only Two Delay Lines. It does not apply to multiple simultaneous input signals.
A Sampling Spectrum Analyzer appeared in U.S. Pat. No. 4,504,785 granted to T. W. Tucker and L. J. Conway in 1985 which utilizes tapped delay lines and a distributed sampling concept to compute a Discrete Fourier Transform (DFT) of a uniform set of samples of an RF input. Although the Sampling Spectrum Analyzer may receive simultaneous signals and measure multiple frequencies in principle, the implementation difficulties are great, and achievable performance (bandwidth and measurement accuracy) for a given number of delays is very limited, compared to the present invention.
Perhaps the simplest IFM circuit, utilizing a single 90.degree. hybrid, appeared in U.S. Pat. No. 4,414,505 granted to H. C. Cuckson and P. D. Curtis in 1983 for Microwave Instantaneous Frequency Measurement Apparatus. The method is intended to measure only a single input frequency.
An IFM Receiver with Digital Processing appeared in U.S. Pat. No. 4,633,516 granted to J. B. Tsui in 1986 which replaces the IFM delay line correlator with a 90.degree. hybrid, A/D circuits and digital processing. Samples arc to be taken at two times differing by .tau.. The processing permits measurement of a single frequency.
An IFM Receiver with Bandwidth Improvement through Phase Shifted Sampling of Real Signals appeared in U.S. Pat. No. 5,109,188 granted to R. B. Sanderson and J. B-Y. Tsui in 1992 which measures the phase of a signal corresponding to a known delay .tau. but in a completely digital system requiring high sample rate A/D converters and other limitations compared to the present invention.
While the IFM receiver has many advantages, a conventional system of the prior art is not intended to measure simultaneously received frequencies. Commercially available IFM receiver systems of the prior art are able to measure the frequency of at most one (the sufficiently stronger) of simultaneously received signals except for certain restricted special cases. It is fairly common for modern radars to transmit pulsed signals consisting of two or more frequencies simultaneously. Furthermore, simultaneous reception of the signals of several radars as coincidental `pulse on pulse` and `pulse on CW` are commonly encountered. Responding to such a signals, the conventional IFM receiver of the prior art would, at best, be able to measure one (the strongest) of the signal frequencies received.
An analytic approach to computing the frequencies of multiple sinusoids comprising a (strictly) uniformly sampled time series obtained, in effect, using an ideal (nondispersive/lossless) tapped delay line,is given in
[1] V. F. Pisarenko, "The Retrieval of Harmonies from a Covariance Function," Geophy. J. Royal Astron. Soc., no. 33, 1973, pp. 347-3663. PA1 [2] R. O. Schmidt, "Multiple Emitter Location and Signal Parameter Estimation," Proc. RADC Spectral Estimation Workshop, Rome Air Development Center, Griffiss AFB, Rome, N.Y. pp. 243-258, 1979. Reprinted in IEEE Trans Antenn Propag, vol. AP-34, no. 3, pp. 276-280, March 1986, and also in Modern Spectrum Analysis II, S. Kesler, Ed., IEEE Press, New York, pp. 141-156, 1986. PA1 R=m.times.m `correlation` matrix, PA1 .lambda..sub.i (R)=ith eigenvalue of R; .lambda..sub.1 .gtoreq..lambda..sub.2 .gtoreq.. . . .gtoreq..lambda..sub.i .gtoreq.. . . .gtoreq..lambda..sub.m PA1 e.sub.i (R)=eigenvector corresponding to eigenvalue .lambda..sub.i, PA1 e.sub.max =eigenvector corresponding to the maximum eigenvalue .lambda..sub.1, PA1 e.sub.min =eigenvector corresponding to the minimum eigenvalue .lambda..sub.m, PA1 1. The time delay discriminator of the IFM prior art but with T nanosecond (lossless or lossy) integration to comprise a time delay correlator. PA1 2. Direct (amplitude) digitization of said correlator outputs to replace the angle (frequency) digitization of the IFM prior art. PA1 3. A modified correlation matrix R--zeros along the main diagonal and time-shifted definitions of correlations in the off-diagonal elements--specifically for use with parallel delay lines instead of the serial (tapped) delay lines of the MUSIC method [1]. A greater number of physical delay lines are generally necessary; the additional delays are always determined by the given delays of the serial (tapped) delay line model. PA1 4. An operational algorithm for computing the frequencies of multiple signals which requires only the minimum eigenvector of said matrix R. Implied and included is a calibration algorithm for "calibrating" the delay line correlators, by calculating and storing the maximum eigenvector of said matrix R. R is measured while the correlators are responding to a single frequency calibration signal from an RF test generator or its equivalent. The calibration eigenvectors corresponding to selected calibration frequencies over the IFM bandwidth are stored for use thereafter in the operational algorithm. PA1 5. If T is Toeplitz, an alternative operational algorithm becomes available; calibration is unnecessary, the minimum eigenvector of B is interpreted as a vector with components which are the coefficients of a polynomial whose roots are to be computed, revealing the frequencies in their arguments. In the MUSIC method, R is, at best, approximately Toeplitz if there is any noise, received or otherwise, in the system. PA1 6. A complete set of delay line correlators has an unambiguous frequency range (bandwidth) and a frequency measurement accuracy. The frequencies measured by the wider bandwidth complete sets may be used to resolve the ambiguities of the narrower so that the accuracy of the narrower may be realized. This is precisely what is traditionally done in the 1 signal case in the prior art IFM. PA1 1. [1 63]is a 2 Signal IFM design using a complete set of 3 delay line correlators with delays .tau.=1,62,63.times..tau..sub.0. It provides the same effective frequency resolution as a 64 point Discrete Fourier Transform (DFT). A practical number of frequency cells is 4096, since one IFM correlator can provide as much as 6 bits beyond the 6 bits provided by the DFT resolution. Two frequencies differing by 16 cells or more (a quarter of the DFT resolution) will each be measured with little or no cross effects. PA1 2. [1 8 63] is a 3 Signal IFM design using 1 complete set of 6 delays .tau.=1,7,8,55, 62,63.times..tau..sub.0. Again, it provides the same effective frequency resolution as a 64 point DFT. A practical number of frequency cells is 4096. Two frequencies differing by 16 cells or more will each be measured with little or no cross effects and there is a capability for measuring 3 simultaneous frequencies. PA1 3. [1][4][16][64] is a 1 signal IFM design using 4 sets (indeed, a well-known traditional design). Each complete set is equivalent to a single delay line correlator. A practical number of frequency cells is 4096. Two frequencies, simultaneously received, cannot be measured with this design. PA1 4. [1 2][2 4][4 8][16][16 32][32 64] is a 2 signal IFM design using 6 complete sets for a total of 7 delays .tau.=1,2,4,8,16,32,64.times..tau..sub.0. Each complete set utilizes 3 delay line correlators. The design will be close in performance to the traditional 1 signal, 7 cascade IFM design--[1][2][4][8][16][32][64]--but with 2 frequency measurement capability. A practical number of frequency cells is 4096. Two frequencies differing by 16 cells or more will each be measured with little or no cross effects. PA1 5. [1 32][2 64] is a 2 signal IFM design using 2 complete sets for a total of 6 delays .tau.=1,2,31,32,62,64.times..tau..sub.0. A practical number of frequency cells is 4096. Two frequencies differing by 16 cells or more will each be measured with little or no cross effects. PA1 6. [16 32 48] is a 3 signal IFM design using a single complete set of 4 uniformly-spaced delays .tau.=16, 32, 48 .times..tau..sub.0. The frequency resolution is the same as the above designs, but the unambiguous bandwidth of this design is 1/16th that of the above designs. It is a Toeplitz design; if the delay lines arc ideal (nondispersive/lossless), it may be implemented without calibration, using a polynomial root algorithm (see ifm2.m) for frequency. If the delay lines are not nearly ideal, the general algorithm (see ifml.m and calib.m) must be used. This design provides the same frequency resolution as a 4 point DFT! A practical number of frequency cells is only 256; this design has only 1/16th the unambiguous bandwidth of designs 1 and 2 above.
A method of computing the directions of arrival of multiple signals using arbitrarily placed antennas with arbitrary directional responses and which also applies to measuring frequencies of multiple sinusoids comprising a sampled data time series, is well known as the MUltiple SIgnal Classification (MUSIC) algorithm. It has been successfully implemented for direction finding at frequencies up to 2 GHz but which, when used to measure frequency, is directly suited to tapped delay line implementations. MUSIC is described in
The MUSIC algorithm applies to digitized data simultaneously sampled at m taps of a tapped delay line through which the signal is passing. The taps need not be uniformly spaced. The algorithm makes use of all m(m-1)/2 correlations r.sub.ij between the video signals at the ith and jth taps and the (auto)correlations r.sub.ii for a total of m(m+1)/2 correlations. The MUSIC technique has been widely known since 1979 and has not been applied successfully to the simultaneous signals IFM problem addressed by the present invention.
There are no commercially available IFM receivers capable of measuring and reporting multiple frequencies of simultaneously received signals.